encryptor: add support for symmetric encryption

[exg]

This change adds variants of the Encryptor and Decryptor classes that use symmetric encryption.

[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (e7d8d4a) 71.97% compared to head (3e3b1f9) 72.30%.

Files	Patch %	Lines
rohmu/encryptor.py	89.54%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   71.97%   72.30%   +0.33%     
==========================================
  Files          35       35              
  Lines        3928     4015      +87     
==========================================
+ Hits         2827     2903      +76     
- Misses       1101     1112      +11     

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[rikonen]

Looks good to me. Leaving open for other people to have a look.

For context, we need this for AR-479 (internal ticket) where we want to use (unique) AES keys directly without any intermediate RSA.

[c-garcia]

LGTM